Interest in amine functional polymers has grown rapidly in recent years owing to their demonstrated utility in a broad spectrum of operations which capitalize on their solubility in water and their reactivity under relatively mild conditions. Such polymers have found uses as flocculants, filtration aids, paper strengthening agents, in enhanced oil recovery and as crosslinking agents for epoxy resins and polyurethanes. As the commercial value of these polymers has become recognized, more and more attention has been devoted to developing polymers containing amine functionality but tailored to specific needs.
Poly(vinylamine) (pVA) is perhaps the most promising of amine functional polymers because of its simplicity of manufacture and its versatility. Since its theoretical monomeric unit, vinylamine, is unstable, pVA is made indirectly by polymerizing a vinylamide, such as N-vinylformamide, and hydrolyzing the resulting polymer to develop its amine functionality. Complete hydrolysis is difficult if not impossible to achieve and it is well recognized that polymers made by this route and referred to as homopolymers of vinylamine are actually copolymers containing vinylamine units (perhaps as high as 98 mol percent) and N-vinylformamide units. A third unit, amidine, has also been recognized as an impurity which is to be minimized in order to increase primary amine functionality.
It has been known for over a decade that hydrochloric acid hydrolysis of poly(N-vinylacetamide) does not produce an absolute homopolymer but a product containing both amine and amidine units resulting from the acid-catalyzed condensation of adjacent amine and acetamido groups. See J. P. Brown et al., Journal of Medicinal Chemistry, 26, 1300 (1983). This article, citing unpublished work of Dawson and Brock, states on page 1304 that the amidine formation can be avoided by use of alkaline hydrolysis media.
U.S. Pat. No. 4,393,174, Dawson et al. (1983) expands on the Brown et al. disclosure in describing the preparation of poly(N-vinylacetamide) and poly(N-vinylformamide) with subsequent hydrolysis to poly(vinylamine) which is useful in making polymeric dyes. It is pointed out that hydrolysis of the amide groups is not easy and has typically been carried out in refluxing aqueous hydrochloric acid. Such conditions are said to result in amidine formation as an impurity which can be avoided by carrying out the hydrolysis at temperatures between 110.degree. and 170.degree. C. in an aqueous strong base such as NaOH.
U.S. Pat. No. 4,421,602, Brunnmueller et al., (1983) discloses making homopolymers of N-vinylformamide which are partially hydrolyzed so that from 10 to 90% of the formyl groups are split off to obtain a polymer containing 90 to 10 mol percent vinylamine units and 10 to 90 mol percent N-vinylamide units i n random distribution. The product described has no amidine units. Hydrolysis conditions disclosed involve the use of acids or bases at temperatures from 20.degree. to 200.degree. C. Particularly preferred temperatures are 70.degree. to 90.degree. C., which is the range within which all the operative examples fall. When using acid hydrolysis, exemplified by hydrochloric acid, the pH is 0 to 5. In an alkaline medium, exemplified by 10% sodium hydroxide solution, the pH is 9 to 14. It is stated that it is also possible to use ammonia, an amine or an alkaline earth metal base such as calcium hydroxide, or aqueous solutions of ammonia or an amine. This is not demonstrated, but it states that if the solvolysis is carried out in ammonia or an amine, formamide or a substituted formamide is obtained as a by-product.
The use of ammonia or a primary or secondary amine in the manufacture of water soluble poly(vinylamine) is disclosed in Japanese Laid-Open Patent Application No. 61-118406 (1986), but in this procedure ammonia or amine is used as a purification aid prior to base hydrolysis using a strongly basic material, preferably sodium or potassium hydroxide, at 20.degree. to 100.degree. C. There is no indication of amidine formation.
While it is recognized that carboxamides can be hydrolyzed under either acidic or basic conditions to amines, from a practical point of view it is said to be advantageous to hydrolyze amides under acidic catalysis, using conditions normally employed for protein degradation. See Physical Organic Chemistry, N. S. Isaacs, John Wiley and Sons, New York, (1987) pp. 484-485. In either case, however, when applied to the conversion of poly(N-vinylformamide) (pNVF), inorganic coproducts are formed along with the poly(vinylamine). Base hydrolysis leads to alkali metal salts, such as sodium or potassium formate, while acid hydrolysis gives the corresponding salt of poly(vinylamine) and formic acid. Neutralization provides poly(vinylamine) (pVA) accompanied by a salt of the acid used for hydrolysis and, unless formic acid has been removed, a formate salt.
The desirability of a salt-free product is recognized in U.S. Pat. No. 4,943,676, Pinschmidt, Jr. et al. (1990). The problem is addressed by avoiding the hydrolysis procedure altogether. As disclosed in this patent, pNVF is subjected to a thermolytic reaction by heating to a temperature sufficient to effect thermodecarbonylation and yield a polymer containing free amine functionality. This product also contains amidine linkages formed from adjacent formamide and amine groups with loss of water. The polymer product disclosed contains randomly linked units of vinylamine, amidine and N-vinylformamide. The amidine units are said to be unwanted and can be suppressed by the presence of water which drives the equilibrium reaction back to the amine and formamide moieties.
In a field of chemistry unrelated to amine functional polymers, ammonium formate has been recognized as a suitable hydrogen donor in catalyzed hydrogenation and dehalogenation reactions. For example, M. K. Anwer, et al., J. Org. Chem., 54, 1284, (1989) describes the use of various palladium on carbon catalysts to effect dehalogenation of polychlorinated aryl compounds with ammonium formate serving as an in situ source of hydrogen. The efficacy of hydrogen transfer is said to increase with increased palladium loading on the support.
H. Wiener, et al., J. Org. Chem., 56, 4481 and 6145, (1991) present two articles on the use of formate salts in transfer reactions catalyzed with palladium supported on carbon. The first of these papers deals with hydrogenation of nitroarenes and the second with hydrogenation of aryl halides. Potassium and sodium formates are the preferred salts for this hydrogen transfer service. Ammonium formate is said to be less effective hydrogen donor, particularly for large scale synthesis.
U.S. Pat. No. 5,099,067, A. G. M. Barrett, et al., (1992), on the other hand, discloses conversion of nitro alcohols to hydroxy amines using ammonium formate with a carbon-supported palladium catalyst. Operable catalysts are said to be any of the conventional hydrogenation-dehydrogenation catalysts which include supported noble metals such as ruthenium, palladium or rhodium. In addition to carbon, suitable supports include clay, alumina and silica.
There has apparently been no recognition in the art that these hydrogen transfer or hydrogenation-dehydrogenation catalysts might be useful in promoting the elimination of troublesome formate salt forming impurities from certain products.